Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Paper No.J-230.)
GEOTEXTILE AS REINFORCEMENT IN FLEXIBLE PAVEMENT FOR SWELLING
SUBGRADE
R. M. Tailor, Assistant Prof, CED, S.V. National Institute of Technology, Surat-395007, email: [email protected],
M.D. Desai, Visiting Prof, AMD, S.V. National Institute of Technology, Surat-395007, email: [email protected],
N. C. Shah, Professor, CED, S.V. National Institute of Technology, Surat-395007, email: [email protected],
ABSTRACT: The South Gujarat region in India have majority of top soil as black cotton soil. The black cotton soil has
characteristics of shrinking on drying and heaving on wetting. This soil being expansive creates several types of damages to
pavement structures, and in some cases the pavement may even become unserviceable. The compacted black cotton
subgrade undergo volumetric changes due to seasonal variation. The normal climate condition of study area shows short
wet and long dry period which aggravate the problem of swelling and shrinkage. The IRC: 37 – 2001, Annexure – 4
suggest 0.6 to 1.0 m thick non-cohesive soil cushion on the expansive soil for road construction which led to higher cost for
road construction. Also for new urban areas it is difficult to raise the embankment or to excavate the subgrade upto such a
depth due to existing structures and under laying service lines. To provide economical solution along with feasible
application Geotextile used as reinforcement material for flexible pavement. It is provided below the pavement components
to act against the heaving of the swelling soil at the same time it helps as drainage layer also. Field study is undertaken to
observe the effect of Geotextile in flexible pavement performance and about 2 to 3 boundary conditions are created for
observations. Observations are in progress for positive results of performance of paved road reinforced with Geotextile
subjected to drying and wetting cycles.
INTRODUCTION
Roads are vital to link our communities and sustain the
economy and quality of life in society. Roads constructed
over the expansive soil observed with high maintenance
expenditure inspite of high capital cost. As per Austroads -
2002 [1] construction and maintenance works on pavements
in Australia and New Zealand cost three billion per year, or
approximately half of the total annual road expenditure.
These are because many roads in this region are failing
prematurely due to the expansion of reactive soils
underneath the roadway, causing safety issues and increases
road maintenance costs.
Frost, Fleming and Rogers [2] outline the primary roles that
a subgrade or pavement foundation must play in pavement
design. The volume change at subgrade creates variety of
failure in flexible pavement like cracking, rutting, potholes
etc. Expansive soils are one of the most problematic
materials that are widely encountered in significant Land
areas in several parts of the world e.g. parts of Africa,
Australia, India, United States and Canada. In these
countries expansive soil is having great impact on the
construction and maintenance costs of highways.
The South West region of India is covered by top soil as
black cotton soil. Figure 1 shows the top soil layer for
India. The map shows that the majority of South Gujarat
area having black cottons soil as top layer.
To understand the phenomenon of expansion of swelling
soil and to provide economical solution along with feasible
application utilising various strength of Geotextiles study
started at the SVNIT campus, South Gujarat region of
India. Geotextile is provided below the pavement
components to act against the heaving of the swelling soil
at the same time it helps as drainage layer also. Field study
is undertaken to observe the effect of geotextile in flexible
pavement performance and about 2 to 3 boundary
conditions are created for observations. Observations are in
progress for positive results of performance of paved road
reinforced with geotextile subjected to drying and wetting
cycles
Fig. 1 Map showing the soil deposits in India.
EXPANSIVE SOILS
Expansive soils are clayey soils, mudstones or shales that
are characterized by their potential for volume change on
drying and/or wetting. Usually the clay content is relatively
high and the clay mineral montmorillonite dominates. They
are characterized by their high strength when dry; very low
strength when wet; wide and deep shrinkage cracks in the
dry season; high plasticity and very poor trafficability when
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Tailor R. M., Desai M. D. & Shah N.C.
wetted. Whenever insufficient attention is given to the
deleterious properties of expansive soils, the results will be
premature pavement failure evidenced by undulations,
cracks, potholes and heave. Methods were developed for
the identification and classification of expansive soils both
locally and worldwide. In India IS: 1498-1970 [3] describe
the methods to identify the expansive soil.
There are three basic particle size components of naturally
occurring soil: sand, silt and clay. Plastic clays termed as
expansive soils or active soils exhibit volume change when
subjected to moisture variations [4]. Swelling or expansive
clay soils are those that contain swelling clay minerals
(such as montmorillonite and smectite) and can often be
scientifically referred to as Vertosols. Vertosols are soils
that contain clay minerals which, because of their natural
physiochemical properties, posses a net negative electrical
charge imbalance that attracts the positive pole of dipolar
water molecules and cations [5]. In addition, expansive
soils have high degree of shrink-swell reversibility with
change in moisture content. Petry and Little [6] discuss the
history of clays and their engineering significance, dating
back to papers written in the early 1930’s.
The effects on buildings constructed on reactive soils with
inadequate footings can be dramatic [7]. Road subgrades
can be viewed as the footings/ foundations for road
pavements, and if these footings are not adequate, structural
damage can occur.
FACTORS INFLUENCING PERFORMANCE OF
PAVEMENTS ON EXPANSIVE CLAYS
Expansiveness is a property of soil influenced by seasonal
climatic conditions, which describes a soil’s propensity to
change in volume with moisture variation. There is no
direct measure of this property due to difficulties in
simulating atmospheric climatic factors, and so it is
necessary to use comparative values of swell measured
under known conditions to assess expansiveness [8].
Damage caused by soil movement is normally restricted to
light structures, such as house slabs, low embankments and
drainage structures. Expansiveness is controlled by three
elements: the type of clay minerals, the change in moisture
content (active depth), and the applied stresses
(embankment loading).
a) Type of Clay Mineral
The type of clay mineral is largely responsible for
determining the intrinsic expansiveness of the soil.
Kaolinitic clays are relatively non-expansive whilst the
more expansive clays are smectite clays, also known as
montmorillonite clays.
b) Active Depth:
Expansive soils will only react if there is a change in
moisture content, to cause either shrinking or swelling. The
change in moisture content (or suction) controls the actual
amount of swell that a particular soil will exhibit under a
particular applied stress. This change in moisture content is
brought about by climatic extremes. The active depth is the
depth over which seasonal moisture changes are observed.
Below this depth, the soil moisture is relatively stable and
therefore volumetrically stable. The active depth can be
estimated by the measurement of soil suction with depth
over time. Pore water suction in soil samples is a more
fundamental and reliable indicator of the degree of
desiccation in an expansive clay profile than the
measurement of moisture content [9]. Figure 2 indicates the
potential active zone of a reactive soil.
Fig. 2 Active Depth for expansive soil [10]
Active depth can be influenced by external factors that are
unrelated to rainfall and runoff. Influential objects such as
trees and urban drainage can cause changes in the active
depth profile, and consequently result in pavement
deformation. Trees cause deep drying of the clay profile by
suction, well beyond the design depth. In addition, trees
produce increased soil moisture changes throughout the
clay profile. This drying often causes significant clay
shrinkage and cracking of road pavements. This is
exacerbated by drought as the tree roots seek moisture from
the clay soils. Climatic variations cause natural variations in
ground moisture. Other factors such as water, sewer or
storm water pipes which leak, cause wetting of soil and
swelling (heave). This is often localized and can distort the
shoulders causing settlement and failures [7].
SUBGRADE TREATMENT METHODS
Petry and Little [6] believe that the majority of treatment
methods currently employed in the field have been around
since 1960; including various forms of chemical or
mechanical modification. The following six methods are
but a few of the popular treatments.
Replacement
Das [11] lists the first precaution of foundation construction
on swelling clays as replacement of the expansive soil with
a less expansive material.
For Indian scenario the IRC: 37-2001 [12] Annexure – 4
suggest 0.6 to 1.0 m thick non-expansive cohesive soil
cushion on the expansive soil for road construction.
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Geotextile as Reinforcement in Flexible Pavement for swelling subgrade
Alternatively insitu Lime - Flyash stabilized soil layer has
been prepared as subgrade.
Compaction
Das [11] states that if clay is compacted at less than OMC,
inter-particle repulsion is minimized and the double layer
surrounding the particle will be suppressed, leading to a
random particle orientation. This means that the soil tends
to swell as there is space for water molecules to occupy,
however, a greater strength is achieved than those soils
compacted greater than OMC. When the soil is on the
“wet” side of OMC, the particles align producing less voids
but a slight reduction in strength
Chemical Stabilization
Generally, there are three types of chemical stabilisers –
traditional, by-product (kiln dust) and non-traditional (such
as sulphonated oils, polymers, enzymes etc). Petry and
Little [6] make the comment that lime and Portland Cement
are the most commonly used chemical stabilisers, however,
moisture stabilisation is still the most widely used method.
Desai M.D. et al [13, 14] conducted research on application
if lime-flyash stabilisation for expansive subgrade at south
Gujarat of India. Results are excellent with reduction in
maintenance cost of the flexible pavement roads within
Surat Municipal Corporation limit.
STUDY AREA OBSERVATIONS
The research started based on the theme to provide effective
solution against the moisture variation and differential
swelling / shrinking of expansive soil in the area. There was
planning of road construction at SVNIT campus near the
observed site. The flexible road was proposed connecting
transportation lab to workshop building on the back side of
Civil Engineering Department. This site was selected for
the further research work. Figure 3 shows the aerial view of
the site as observed in Google web page
Fig. 3 Location of road joining Transportation Engineering
Lab to Workshop, at SVNIT Campus, South Gujarat.
The study by Jigisha [15] shows the average soaked CBR
for typical South Gujarat undisturbed or compacted soil as
2.0 %. The study also describes some ambiguity in results
due to uneven moisture distribution within the soaked test
specimen. Also study carried out by Yogendra [16] shows
the similar type of observation for the South Gujarat region
soil. Table-1 shows the Geotechnical Properties of Black
cotton soil as observed by Yogendra [16].
Table 1- Geotechnical Properties of Black Cotton Soil
Property Values
Gravel (%) 1
Sand (%) 12
Grain Size
Silt + Clay (%) 87
Liquid Limit (%) 55 Atterberg’s Limit
Plasticity Index (%) 27
MDD (kN/cu.m) 15.50 Compaction Test
OMC (%) 21.75
Swelling Test Free Swell Index (%) 70
CBR (%) 1.77
UCS (kN/sq.m) 59
Permeability (m/s) 8.75 x 10-9
Expansive soils react with water and because of the change
in moisture content the soil have active depth varying from
region to region. The study carried out by Desai M.D.
(2011) shows the active depth for South Gujarat region as 3
to 4 m. Some observations for study area expansive soil are
described as Figure 4. It shows the 3 to 4 cm deep crack at
the mid of the parking lot near T.E. Lab which because of
the beneath expansive soil at foundation of slab. The
phenomenon as described by Nelson and Miller [10] that
there will be higher moisture content at inner side of the
slab, the higher moisture content had created heaving at
mid of the parking lot which finally resulted in severe
cracking.
a) Crack at Mid way b) Enlarge view of crack
Fig. 4 Photo showing enlarge view of crack at mid way of
the parking shed.
GEOTEXTILE FOR FOUNDING FLEXIBLE
PAVEMENT
Figure 5 shows the proposed crust composition for road.
The proposed road is studied for its design and planning
was done for the observation of the expansive subgrade
behaviour. After taking necessary approval from authority
it was decided to provide the Geotextile GARWARE made
GWF-52-240 PP Grey Multi 240 Twill 5 M, just below the
subbase layer for further observation. Thick black line in
Figure 5 indicates the geotextile layer as provided in the
road construction.
Fig. 5 Crust composition along with laid geotextile at site,
SVNIT Campus
Proposed Road
Geotextile
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Tailor R. M., Desai M. D. & Shah N.C.
The geotextile was laid in such a manner that 2 boundary
conditions can be created for the site.
1) Road with both side covered ground (length between
Transportation Engineering Lab and Water Resources
Engineering Lab)
2) Road with one side covered and one side open ground
(length along WRE lab & after WRE lab).
Figure 6 shows various stages of the road construction at
site.
a) Geotextile above Murrum, b) GSB spreading on
Geotextile
Fig. 6 Road construction work in progress at site
OBSERVATIONS
The following observation was started after finishing of the
pavement construction upto grouting layer.
a) Visual observation for cracks and other changes
b) Ground profile reading to get amount of change in soil
thickness with change in moisture content (i.e. change in
season).
The visual ground observations show that there is definite
improvement in stability of subgrade below pavement
where the Geotextile was laid. The portion with geotextile
shows less undulation at top as well as very few cracks are
observed. The length without Geotextile showed numbers
of cracks and also there was more settlement observed.
CONCLUSION
Pilot initial field observations during execution traffic have
established feasibility of use of geotextile reinforcement in
pavement to control performance. The observations after 2
monsoons will only confirm its capability to replace
cushion of 600 to 1000 mm for expansive subgrades (IRC:
37-2001). Future studies will be assessing minimum tensile
strength of fabric and extra embedment over traffic zone.
REFERENCES
1. Austroads, (2002), Austroads Pavement Strategy 2001-
2004, Austroads Incorporated, Sydney, NSW
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Record 1757, Soils Geology and Foundations,
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pp.100-108
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ACKNOWLEDGEMENT
Authors are thankful to Ms. Jigisha M. Vashi, Ph.D.
Research Scholar for drafting of this work.
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